Ceramic matrix composites generally include a ceramic fiber reinforcement material embedded in a ceramic matrix material. The reinforcement material serves as the load-bearing constituent of the CMC in the event of a matrix crack, while the ceramic matrix protects the reinforcement material, maintains the orientation of its fibers, and serves to dissipate loads to the reinforcement material. Of particular interest to high-temperature applications, such as in gas turbines, are silicon-based composites, which include silicon carbide (SiC) as the matrix and/or reinforcement material.
Different processing methods have been employed in forming CMCs. For example, one approach includes melt infiltration (MI), which employs a molten silicon to infiltrate into a fiber-containing perform. CMCs formed by prepreg MI are generally fully dense, e.g., having generally zero, or less than 3 percent by volume, residual porosity. This very low porosity gives the composite desirable mechanical properties, such as a high proportional limit strength and interlaminar tensile and shear strengths, high thermal conductivity and good oxidation resistance. However, the matrices of MI composites contain a free silicon phase (i.e. elemental silicon or silicon alloy) that limits the use temperature of the system to below that of the melting point of the silicon or silicon alloy, or about 2550 degrees Fahrenheit to 2570 degrees Fahrenheit. Moreover the free silicon phase caused the MI SiC matrix to have relatively poor creep resistance.
Another approach for forming CMCs is chemical vapor infiltration (CVI). CVI is a process whereby a matrix material is infiltrated into a fibrous preform by the use of reactive gases at elevated temperature to form the fiber-reinforced composite. Generally, limitations introduced by having reactants diffuse into the preform and by-product gases diffusing out of the perform result in relatively high residual porosity of between about 10 percent and about 15 percent in the composite. In particular, typically in forming CMCs using CVI, the inner portion of the composite formed by CVI typically has a higher porosity than the porosity of the outer portion of the composite. The presence of this porosity degrades the in-plane and through-thickness mechanical strength, thermal conductivity, and oxidation resistance of the CVI CMC relative to MI CMCs. However, CVI composite matrices typically have no free silicon phase, and thus have better creep resistance than MI matrices and the potential to operate at temperatures above 2570 degrees Fahrenheit.
Another approach for forming CMCs includes initially employing a partial CVI process followed by a MI process, and is generally referred to as “slurry cast MI”. This approach usually yields an intermediate porosity between that of MI composites and CVI composites, generally of between about 5 percent and about 7 percent, and yields residual free silicon phase within the composite matrix.
There is a need for further ceramic matrix composites (CMC), and more particularly, to articles and methods for forming ceramic matrix composite articles.